Additive for increasing the density of an oil-based fluid and fluid comprising such additive

ABSTRACT

A method of formulating a wellbore fluid that includes grinding a solid particulate material and a polymeric dispersing agent to provide a resulting polymer coated solid material, and suspending the resulting polymer coated solid material in the wellbore fluid. At least a portion of the resulting polymer coated solid material has a particle diameter less than 2.0 microns, wherein the polymeric dispersing agent has a molecular weight greater than 10,000. Exemplary starting materials for the solid material include weighting agents including barite, calcium carbonate, dolomite, ilmenite, hematite or other iron ores, olivine, siderite and strontium sulfate as well as mixture and combinations of these and other similar weighting materials. The dispersant in one illustrative embodiment is a polymeric acrylate ester made from the monomers of stearyl methacrylate, butyacrylate, and acrylic acid.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/737,303, filed Apr. 19, 2007, which is a divisional application ofU.S. application Ser. No. 10/610,499, filed Jun. 30, 2003, now U.S. Pat.No. 7,267,291, which is a continuation-in-part of U.S. patentapplication Ser. No. 09/230,302, filed Sep. 10, 1999, now U.S. Pat. No.6,586,372, which in turn is the U.S. national phase of IntentionalApplication No. PCT/EP97/03802, filed Jul. 16, 1997 which in turnsclaims priority to United Kingdom Application No. GB 9615549.4, filedJul. 24, 1996.

BACKGROUND OF THE INVENTION

One of the most important functions of a wellbore fluid is to contributeto the stability of the well bore, and control the flow of gas, oil orwater from the pores of the formation in order to prevent, for example,the flow or blow out of formation fluids or the collapse of pressuredearth formations. The column of fluid in the hole exerts a hydrostaticpressure proportional to the depth of the hole and the density of thefluid. High-pressure formations may require a fluid with a specificgravity of up to 3.0.

A variety of materials are presently used to increase the density ofwellbore fluids. These include dissolved salts such as sodium chloride,calcium chloride and calcium bromide. Alternatively powdered mineralssuch as barite, calcite and hematite are added to a fluid to form asuspension of increased density. It is also known to utilize finelydivided metal such as iron as a weight material. In this connection, theliterature discloses a drilling fluid where the weight material includesiron/steel ball-shaped particles having a diameter less than 250 μm andpreferentially between 15 and 75 μm. It has also been proposed to usefinely powdered calcium or iron carbonate however the difficulty is thatthe plastic viscosity of such fluids rapidly increases as the particlesize decreases.

It is a requirement of wellbore fluids that the particles form a stablesuspension, and do not readily settle out. A second requirement is thatthe suspension should exhibit a low viscosity in order to facilitatepumping and to minimize the generation of high pressures. Anotherrequirement is that the wellbore fluid slurry should exhibit lowfiltration rates (fluid loss).

Conventional weighting agents such as powdered barite exhibit an averageparticle diameter (d₅₀) in the range of 10-30 μm. To suspend thesematerials adequately requires the addition of a gallant such asbentonite for water-based fluids, or organically modified bentonite foroil based fluids. A soluble polymer viscosifier such as xanthan gum maybe also added to slow the rate of the sedimentation of the weightingagent. However, a penalty is paid in that as more gallant is added toincrease the suspension stability, the fluid viscosity (plasticviscosity) increases undesirably resulting in reduced pumpability. Thisis obviously also the case if a viscosifier is used to maintain adesirably level of solids suspension.

The sedimentation (or “sag”) of particulate weighting agents becomesmore critical in wellbores drilled at high angles from the vertical, inthat sag of, for example, one inch (2.54 cm) can result in a continuouscolumn of reduced density fluid along the upper portion of the wellborewall. Such high angle wells are frequently drilled over large distancesin order to access, for example, remote portions of an oil reservoir. Insuch instances it is important to minimize a drilling fluid's plasticviscosity in order to reduce the pressure losses over the boreholelength. At the same time a high density also should be maintained toprevent a blow out. Further, as noted above with particulate weightingmaterials the issues of sag become increasingly important to avoiddifferential sticking or the settling out of the particulate weightingagents on the low side of the wellbore.

Being able to formulate a drilling fluid having a high density and a lowplastic viscosity is no less important in deep high pressure wells wherehigh-density wellbore fluids are required. High viscosities can resultin an increase in pressure at the bottom of the hole under pumpingconditions. This increase in “Equivalent Circulating Density” can resultin opening fractures in the formation, and serious losses of thewellbore fluid into the fractured formation. Again, however, thestability of the suspension is important in order to maintain thehydrostatic head to avoid a blow out. The objectives of high-densityfluids with low viscosity plus minimal sag of weighting material can bedifficult to reconcile. The need therefore exists for materials toincrease fluid density that simultaneously provide improved suspensionstability and less viscosity increase.

SUMMARY OF THE INVENTION

The claimed subject matter is generally directed to a drilling fluidadditive and a method of making the additive for increasing the densityof a fluid while at the same time maintaining a useful suspensionstability without a significant viscosity increase. In one illustrativeembodiment, the method includes comminuting a solid material and adispersant in a liquid medium, so as to produce solid colloidalparticles that are coated with the dispersant. Preferably the colloidalparticles have a weight average particle diameter (D₅₀) of less thanabout 10 μm and more preferably less than about 2 μm. The liquid mediumis preferably an oleaginous fluid and more preferably an oleaginousliquid that is environmentally acceptable as the continuous phase of anoil based drilling fluid. In order to achieve an optimal and safegrinding process the oleaginous fluid preferably has a kinematicviscosity less than 10 centistokes (10 mm²/s) at 40° C. and a flashpoint of greater than 60° C. Illustrative examples of such oleaginousfluids include diesel oil, mineral or white oils, n-alkanes or syntheticoils such as alpha-olefin oils, ester oils or poly(alpha-olefins), aswell as combinations and mixtures of these and similar fluids whichshould be know to one of skill in the art. The dispersant that is coatedonto the solid particle during the course of grinding is, in oneillustrative embodiment, selected from carboxylic acids of molecularweight of at least 150 Daltons. Alternatively, the dispersant coatingmay be made of compounds including oleic acid, polybasic fatty acids,alkylbenzene sulfonic acids, alkane sulfonic acids, linear alpha-olefinsulfonic acid or the alkaline earth metal salts of any of the aboveacids, and phospholipids as well as mixtures and combinations of thesecompounds. In another alternative and illustrative embodiment thedispersant is a polymeric compound, preferably a polyacrylate ester. Theillustrative polymeric dispersant should have an average molecularweight from about 10,000 Daltons to about 200,000 Daltons and morepreferably from about 17,000 Daltons to about 30,000 Daltons. The solidmaterial may be selected from a wide variety of known weightingmaterials and in one illustrative embodiment the solids material isselected from the group consisting of barite, calcium carbonate,dolomite, ilmenite, hematite or other iron ores, olivine, siderite, andstrontium sulfate, mixtures and combinations of these and similarweighting materials that should be known to one of skill in the art. Inone preferred illustrative embodiment, the comminuting of the solidmaterial and the dispersant in the liquid medium is carried out in anagitated fluidized bed of particulate grinding material.

These and other features of the claimed subject matter are more fullyset forth in the following description of preferred or illustrativeembodiments of the invention.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

It is known in the art that reduced particle sedimentation rates can beobtained by reducing the particle size used. However, the conventionalview in the drilling industry is that reducing the particle size causesan undesirable increase in viscosity. The rapid increase in viscosity asparticle size decreases is believed to be caused by an increase in thesurface area of the particles causing the increased adsorption of wateronto the surface of the particles. As reported in, “Drilling andDrilling Fluids” Chilingarian G. V. and Vorabutor P. 1981, pages441-444, “The difference in results (i.e. increase in plastic viscosity)when particle size is varied in a mud slurry is primarily due tomagnitude of the surface area, which determines the degree of adsorption(tying up) of water. More water is adsorbed with increasing area.”Further it is also stated “Viscosity considerations often will notpermit the addition of any more of the colloidal solids necessary tocontrol filtration, unless the total solids surface area is firstreduced by removing a portion of the existing clays”. Thus it isreported in the literature that colloidal fines, because of their highsurface area to volume ratio, will adsorb significantly more drillingfluid than and larger particles. Because of this high adsorption ofdrilling fluid to the surface of the particle, an increase in theviscosity (i.e. a decrease in the fluidity) of the mud is observed. Forthese reasons, one of skill in the art should understand and appreciatethat it is necessary in weighted particulate muds to remove the finesolids to reduce the viscosity increase cause by such fine particles.This concept is reflected in the API specification for barite as adrilling fluid additive. As indicated in the API specification the % w/wof particles having a diameter below 6 μm is limited to 30% w/w maximumin order to minimize viscosity increases.

In view of the above, one of skill in the art should immediatelyappreciate and understand that it is very surprising that the productsof this invention, which utilize particles ground to an average particlediameter (d₅₀) of less than 2 μm, provide wellbore fluids of reducedplastic viscosity while at the same time greatly reducing sedimentationor sag.

The additives of this invention comprise solid colloidal particles witha defloculating agent or dispersant coated onto the surface of theparticle. The fine particle size generates high density suspensions orslurries that show a reduced tendency to sediment or sag, while thedispersant on the surface of the particle controls the inter-particleinteractions resulting in lower rheological profiles. Thus it is thecombination of high density, fine particle size and control of colloidalinteractions by surface coating the particles with a dispersant thereconciles the objectives of high density, lower viscosity and minimalsag.

One of skill in the art will appreciate and understand that the use ofsmall particles in drilling fluids is well known in the art, but for atotally different purpose. For example, in EP-A-119 745 an high-densityfluid for blow-out prevention is disclosed that contains water, a firstand possibly second weighting agent and a gellant made of fine particles(average diameter from 0.5 to 10 μm). The gelling agent particles aresmall enough to impart static gel strength to the fluid by virtue of theinterparticle attractive forces. One of skill in the art should alsoappreciate that if the concentration of weighting agent is sufficientlylow, no gelling agent is needed in the fluids of EP-A-119 745. Thus, thesmall particle size imparts to the fluids of EP-A_(—)119 745 theviscosifier properties that result from the high surface area to volumeratio of the small particles. The teachings of the EP-A-119 745reference and other similar references are exactly opposite to those ofthe claimed subject matter. That is to say, the teachings of the priorart indicate that small particle size material can be added to adrilling fluid, but that doing so increases the viscosity of thedrilling fluid. In contrast, the surprising results of the claimedsubject matter is that one can add very fine particulate material thatis coated with a dispersant layer and not have the rapid increases inviscosity exhibited by the prior art.

According to the claimed subject matter, a dispersant is coated onto theparticulate weighting additive during the comminution (grinding)process. That is to say, coarse weighting additive is ground in thepresence of a relatively high concentration of dispersant such that thenewly formed surfaces of the fine particles are exposed to and thuscoated by the dispersant. It is speculated that this allows thedispersant to find an acceptable conformation on the particle surfacethus coating the surface. Alternatively it is speculated that because arelatively higher concentration of dispersant in the grinding fluid, asopposed to that in a drilling fluid, the dispersant is more likely to beabsorbed (either physically or chemically) to the particle surface. Asthat term is used in herein, “coating of the surface” is intended tomean that a sufficient number of dispersant molecules are absorbed(physically or chemically) or otherwise closely associated with thesurface of the particles so that the fine particles of material do notcause the rapid rise in viscosity observed in the prior art. By usingsuch a definition, one of skill in the art should understand andappreciate that the dispersant molecules may not actually be fullycovering the particle surface and that quantification of the number ofmolecules is very difficult. Therefore by necessity reliance is made ona results oriented definition. As a result of the inventive process,Applicants have discovered that one can control the colloidalinteractions of the fine particles by coating the particle withdispersants prior to addition to the drilling fluid. By doing so, it ispossible to systematically control the rheological properties of fluidscontaining in the additive as well as the tolerance to contaminants inthe fluid in addition to enhancing the fluid loss (filtration)properties of the fluid.

Evidence in support of the results oriented definition above can befound in the working examples below as well as the prior art. As is wellknow to one of skill in the art, in the absence of the coatingdispersant, a concentrated slurry of particles having a d₅₀ of less than2 μm, will result in an unpumpable paste or gel. According to the methodand compositions of the claimed subject matter, a dispersant is coatedonto the particle surface during the grinding or comminution process.This provides an advantageous improvement in the state of dispersion ofthe particles compared to post addition of the dispersant to fineparticles. The presence of the dispersant in the comminution processyields discrete particles which can form a more efficiently packedfilter cake and so advantageously reduce filtration rates.

According to one illustrative embodiment, the dispersant is chosen sothat it provides the suitable colloidal inter-particle interactionmechanism to make it tolerant to a range of common wellborecontaminants, including salt saturation.

According to a preferred embodiment of the claimed subject matter, theweighting agent of the claimed subject matter is formed of particlesthat are composed of a material of specific gravity of at least 2.68.This allows wellbore fluids to be formulated to meet most densityrequirements yet have a particulate volume fraction low enough for thefluid to be pumpable.

A preferred embodiment of this invention is for the weight averageparticle diameter (d₅₀) of the new weighting agent to be less than 1.5micron. This will enhance the suspension's characteristics in terms ofsedimentation or sag stability without the viscosity of the fluidincreasing so as to make it unpumpable.

A method of comminuting a solid material to obtain material containingat least 60% by weight of particles smaller than 2 μm is known forexample from British Patent Specification No. 1,472,701 or No.1,599,632. As is taught therein, the coarse mineral in an aqueoussuspension is ground within an agitated fluidized bed of a particulategrinding medium for a time sufficient to provide the required particlesize distribution. The same process of grinding can be carried out bysubstituting an oleaginous (oil) based fluid for the aqueous basedfluid. An important preferred embodiment aspect of the claimed subjectmatter is the presence of the dispersing agent in the step of “wet”grinding the mineral.

The colloidal particles may be provided as a concentrated slurry eitherin an aqueous medium or more preferably as an organic liquid. In thelatter case, the organic liquid should be acceptable as a component andhave the necessary environmental characteristics required for additivesto oil-based drilling fluids. With this in mind it is preferred that theoleaginous fluid have a kinematic viscosity of less than 10 centistokes(10 mm2/s) at 40° C. and, for safety reasons, a flash point of greaterthan 60° C. Suitable oleaginous liquids are for example diesel oil,mineral or white oils, n-alkanes or synthetic oils such as alpha-olefinoils, ester oils or poly(alpha-olefins), mixtures of these fluids aswell as other similar fluids which should be well known to one of skillin the art of drilling fluid formulation.

When the colloidal particles are provided in an aqueous medium, thedispersing agent may be, for example, a water soluble polymer ofmolecular weight of at least 2,000 Daltons. The polymer is a homopolymeror copolymer of any monomers selected from (but not limited to) theclass comprising: acrylic acid, itaconic acid, maleic acid or anhydride,hydroxypropyl acrylate vinylsulphonic acid, acrylamido 2-propanesulphonic acid, acrylamide, styrene sulphonic acid, acrylic phosphateesters, methyl vinyl ether and vinyl acetate. The acid monomers may alsobe neutralised to a salt such as the sodium salt.

It is known that high molecular weight polymers act as flocculants bybridging between particles while low molecular weight polymers forinstance (less than 10,000) act as deflocculants by creating overallnegative charges.

It has been found that when the dispersing agent is added whilegrinding, intermediate molecular weight polymers (in the range 10,000 to200,000 for example) may be used effectively. Intermediate molecularweight dispersing agents are advantageously less sensitive tocontaminants such as salt and therefore are well adapted to wellborefluids.

Where the colloidal particles are provided in an organic medium, thedispersing agent may be selected for example among carboxylic acids ofmolecular weight of at least 150 such as oleic acid and polybasic fattyacids, alkylbenzene sulphonic acids, alkane sulphonic acids, linearalpha-olefin sulphonic acid or the alkaline earth metal salts of any ofthe above acids, phospholipids such as lecithin, as well as similarcompounds that should be readily apparent to one of skill in the art.Synthetic polymers may also be utilized such as Hypermer OM-1 (trademarkof ICI) or alternatively polyacrylate esters. However, one of skill inthe art should appreciate that other acrylate monomers may be used toachieve substantially the same results as disclosed herein. Theillustrative polymeric dispersant should have an average molecularweight from about 10,000 Daltons to about 200,000 Daltons and morepreferably from about 17,000 Daltons to about 30,000 Daltons.

The colloidal particles are themselves composed of weighting materialsthat are well known to one of skill in the art of weighting drillingfluids. In one illustrative embodiment, the particles are made from oneor more materials selected from but not limited to barium sulphate(barite), calcium carbonate, dolomite, ilmenite, hematite or other ironores, olivine, siderite, strontium sulphate. Normally the lowestwellbore fluid viscosity at any particular density is obtained by usingthe highest density colloidal particles. However other considerationsmay influence the choice of product such as cost, local availability andthe power required for grinding. Minerals such as calcium carbonate anddolomite posses the advantage that residual solids or filter cake may bereadily removed from a well by treatment with acids.

The compositions resulting from the methods of the claimed subjectmatter have a surprising variety of applications in drilling fluids,cement, high density fluids and coiled tubing drilling fluids tohighlight a few. The new particulate weighting agents have the abilityto stabilize the laminar flow regime, and delay the onset of turbulence.It is possible to formulate fluids for several applications includingcoiled tubing drilling fluids that will be able to be pumped fasterbefore turbulence is encountered, so giving essentially lower pressuredrops at equivalent flow rates. The ability to stabilize the laminarflow regime although surprising, is adequately demonstrated in heavydensity muds of 20 pounds per gallon (2.39 g/cm³) or higher. Such highdensity muds using conventional weighting agents with a weight averageparticle diameter of 10 to 30 μm would exhibit dilatancy with theconcomitant increase in the pressure drops due to the turbulencegenerated. The ability of the new weighting agent to stabilize the flowregime even in the presence of a component of larger particles meansthat high-density fluids with acceptable rheology are feasible withlower pressure drops.

A further and unexpected application occurs in cement whereby the newweighting agent will generate slurries of a more controlled and lowerrheology thus allowing the slurry to be pumped more freely intoposition. One of skill in the art should appreciate that the reducedparticle size will tend to have a less abrasive nature, while itssuspension characteristics will reduce the free water and othersuspension issues encountered when setting the cement. The high fractionof fines should also act as efficient fluid loss control agents thuspreventing gas migration and producing stronger cements.

The fluids of the claimed subject matter may also be used innon-oilfield applications such as dense media separating fluid (torecover ore for example) or as a ship's ballast fluid.

The following examples are to illustrate the properties and performanceof the wellbore fluids of the claimed subject matter though theinvention is not limited to the specific embodiments showing theseexamples. All testing was conducted as per API RP 13 B where applicable.Mixing was performed on Silverson L2R or Hamilton Beach Mixers. Theviscosity at various shear rates (RPM's) and other rheologicalproperties were obtained using a Fann viscometer. Mud weight werechecked using a standard mud scale or an analytical balance. Fluid losswas measured with a standard API fluid loss cell.

In expressing a metric equivalent, the following U.S. to metricconversion factors are used: 1 gal=3.785 litres; 1 lb.=0.454 kg; 1lb./gal (ppg)=0.1198 g/cm3; 1 bbl=42 gal; 1 lb./bbl (ppb)=2.835 kg/m³; 1lb/100 ft²=0,4788 Pa.

These tests have been carried out with different grades of barite: astandard grade of API barite, having a weight average particle diameter(D₅₀) of about 20 μm; a commercial barite (M) made by milling/grindingbarite whilst in the dry state, with an average size of 3 μm-5 μm andcolloidal barite according the claimed subject matter (with a D₅₀ from0.5 μm to 1.5 μm), with a dispersant included during the “wet” grindingprocess. The corresponding particle size distributions are shown FIG. 1.The dispersant is IDSPERSE™ XT (Mark of Schlumberger), an anionicacrylic ter-polymer of molecular weight in the range of 40,000 to120,000 with carboxylate and other functional groups. This preferredpolymer is advantageously stable at temperature up to 200° C., tolerantto a broad range of contaminant, gives good filtration properties and donot readily desorb off the particle surface.

Samples were measured on a Malvern microplus instrument using thepresentation (optical model) R1 (particle 1.61; absorption 0.1; R1(Dispersant) 1.46. The analysis was done using a drop of the groundmaterial in an oil dispersant.

The following examples are included to demonstrate preferred embodimentsof the invention. It should be appreciated by those of skill in the artthat the techniques disclosed in the examples which follow representtechniques discovered by the inventors to function well in the practiceof the invention, and thus can be considered to constitute preferredmodes for its practice. However, those of skill in the art should, inlight of the present disclosure, appreciate that many changes can bemade in the specific embodiments which are disclosed and still obtain alike or similar result without departing from the scope of theinvention.

Example 1

Several 22 ppg [2.63 g/cm³] fluids, based on barium sulphate and water,were prepared using standard barite and colloidal barite according tothe invention. The 22 ppg slurry of API grade barite and water was madewith no gelling agent to control the inter-particle interactions (Fluid#1). Fluid #2 is also based on standard barite but with a post-additionof two pounds per barrel (5.7 kilograms per cubic metre) IDSPERSE XT.Fluid #3 is 100% new weighting agent with 67% w/w of particles below 1micron in size and at least 90% less than 2 μm. The results are providedin table 1.

TABLE I Yield Viscosity at various shear rates (rpm of agitation):Plastic Point Dial reading or “Fann Units” for: Viscosity lb/100 ft² #600 rpm 300 rpm 200 rpm 100 rpm 6 rpm 3 rpm mPa · s (Pascals) 1 250 160124 92 25 16 90 70 (34) 2 265 105 64 26 1 1 160 −55 (−26) 3 65 38 27 173 2 27 11 (5) 

Upon review of the above data one of skill in the art should appreciatethat the viscosity of Fluid #1 is very high and the slurry was observedto filter very rapidly. It should also be appreciated that if furthermaterials were added to reduce the fluid loss, the viscosity wouldincrease yet further. Also notable is that this system sagssignificantly over one hour giving substantial free water (ca. 10% oforiginal volume).

The post addition of two pounds per barrel [5.7 kg/cm³] of IDSPERSE XTto this system (Fluid #2) appears to reduce the low shear rate viscosityby controlling the inter-particle interactions. However it will be notedthat because of the particle concentration and average particle size,the fluid exhibits dilatency, which is indicated by the high plasticviscosity and negative yield point. It should be appreciated that thiswill result in substantial pressure drops during the pumping of thesefluids. Further it should be noted that Fluid #2 sags immediately onstanding.

One of skill in the art should note that with regard to Fluid #3, thefluid exhibits a substantially lower plastic viscosity when compared toFluids #1 and #2. The presence of the dispersing agent coated onto theparticles appears to control the inter-particle interactions, thusmaking fluid #3 pumpable and not gel-like. It should also be appreciatedthat the much lower average particle size has stabilized the flowregime. That is to say a review of the data will reveal that the flow isnow laminar at 1000 s⁻¹ as demonstrated by the low plastic viscosity andpositive yield point.

Upon consideration of the above data, one of skill in the art shouldappreciate that there exists an observable and substantial effect on therheological properties of the above fluids caused by the coating of thefine particles by the dispersant agent. That is to say, the propertiesand results of the claimed invention are achieved when the particles arefirst coated with dispersant and then added to the fluid. This is incontrast to the properties and results achieved when no dispersant isused or when the dispersant is simply added to the drilling fluid alongwith the particles. One of skill in the art of drilling fluidformulation will appreciate that it is wide-spread practice within theindustry to simply combine materials into a base fluid to achieve thedesired final formulation. However, as supported by the above data, thecoating of a dispersant onto fine particulate weighting materials priorto addition to the base fluid results in a substantial and observabledifference in rheological properties that are surprising and unexpected.

Example 2

Experiments were conducted to examine the effect of the post addition ofthe chosen polymer dispersant to a slurry formulated to includeweighting agents of the same colloidal particle size. A milled barite(D₅₀˜4 μm) and a comminuted calcium carbonate (70% by weight of theparticles of less than 2 μm) were selected, both of which are of similarparticle size to materials disclosed herein. The slurries were preparedat an equivalent particle volume fraction of 0.282. See table II.

The rheologies were measured at 120° F. (49° C.), thereafter an additionof 6 ppb (17.2 kg/m³) IDSPERSE XT was made. The rheologies of thesubsequent slurries were finally measured at 120° F. (see table 111)with additional API fluid loss test.

TABLE II Volume # Material Dispersant Density (ppg) Fraction wt/wt 4 NewBarite while grinding 16.0 [1.92 g/cm³] 0.282 0.625 5 Milled Barite none16.0 [1.92 g/cm³] 0.282 0.625 6 Milled Barite post-addition 16.0 [1.92g/cm³] 0.282 0.625 7 Calcium Carbonate none 12.4 [1.48 g/cm³] 0.2820.518 8 Calcium Carbonate post-addition 12.4 [1.48 g/cm³] 0.282 0.518

TABLE III Viscosity at various shear rates (rpm of agitation): PlasticYield API Dial reading or “Fann Units” for: Viscosity Point Fluid # 600rpm 300 rpm 200 rpm 100 rpm 6 rpm 3 rpm mPa · s lb/100 ft² Loss 4 12 6 42 6 0 11 5 os os os os os os 6 12 6 4 2 6 0 total¹ 7 os os 260 221 88 788 12 6 4 3 1 1 6 0 total² ¹total fluid loss in 26 minutes ²total fluidloss in 20 minutes

Upon review of the above data one of skill in the art will note that nofiltration control is gained from post addition of the polymer asrevealed by the total fluid loss in the API test.

Example 3

This test was carried out to show the feasibility of 24 ppg [2.87 g/cm³]slurries (0.577 Volume fraction). Each fluid contained the followingcomponents e.g. Fresh Water 135.4 g, Total Barite 861.0 g, IDSPERSE XT18.0 g. The barite component was varied in composition according to thefollowing table.

TABLE IV API grade Colloidal # Barite (%) Barite (%) 9 100 0 10 90 10 1180 20 12 75 25 13 60 40 14 0 100

TABLE V Yield Viscosity at various shear rates (rpm of agitation):Plastic Point Dial reading or “Fann Units” for: Viscosity lb/100 ft² #600 300 200 117 100 59 30 6 3 mPa · s (Pascals) 9 *os 285 157 66 56 2610 3 2 10 245 109 67 35 16 23 7 3 2 136 −27 (−13) 11 171 78 50 28 23 107 3 2 93 −15 (−7)  12 115 55 36 19 17 8 5 3 2 60 −5 (−2) 13 98 49 34 2120 14 10 4 3 49 0 14 165 84 58 37 32 22 18 5 3 81   3 (−1.5) *os =off-scale

Upon review of the results provided table V one of skill in the artshould appreciate that API grade barite, because of its particle sizeand the high volume fraction required to achieved high mud weights,exhibits dilatancy i.e. high plastic and apparent viscosity and negativeyield values.

Further it should be noted that introduction of fine grade materialstends to stabilize the flow regime keep it laminar at higher shearrates: plastic viscosity decreases markedly and yield point changes fromnegative to positive. In addition it will be noticed that no significantincrease in low-shear rate viscosity (@ 3 rpm) is caused by thecolloidal barite.

The above results will show to one of skill in the art that thecolloidal weight material coated with dispersant as is disclosed hereinmay advantageously be used in conjunction with conventional API barite.

Example 4

An eighteen (18) pound per gallon [2.15 g/cm³] slurry of weighting agentaccording the claimed subject matter was formulated and subsequentlycontaminated with a range of common contaminants and hot rolled at 300°F. (148.9° C.). The rheological results of before (BHR) and after hotrolling (AHR) are presented below.

TABLE VI (New barite) Viscosity (Fann Units) at various YP Fluid shearrates (rpm of agitation: PV lb/100 ft² loss 600 300 200 100 6 3 mPa · s(Pascals) ml no contaminant BHR 21 11 8 4 1 1 10  1(0.5 no contaminantAHR 18 10 7 4 1 1 8 2(1) 5.0 +80 ppb NaCl BHR 41 23 16 10 2 1 18  5(2.5) +80 ppb NaCl AHR 26 14 10 6 1 1 12 2(1) 16 +30 ppb OCMA¹ BHR 3822 15 9 2 1 16 6(3) +30 ppb OCMA AHR 26 14 10 6 1 1 12 2(1) 6.8 +5 ppbLime BHR 15 7 5 3 1 1 8   −1(−0.5) +5 ppb Lime AHR 10 5 4 2 1 1 5 0 6.4¹OCMA = OCMA clay, a fine particle ball clay commonly used to replicatedrilled solids contamination acquired from shale sediments duringdrilling

Upon review of the above results one of skill in the art shouldappreciate that the dispersant coated weight material system showsexcellent resistance to contaminants, low controllable rheology andgives fluid loss control under a standard API mud test as shown infollowing table VI. An equivalent set of fluids were prepared using APIconventional barite without the polymer coating as a direct comparisonof the two particle types. (Table VII)

TABLE VII (Conventional API Barite Viscosity (Fann Units) at various YPFluid shear rates (rpm of agitation: PV lb/100 ft² loss 600 300 200 1006 3 mPa · s (Pascals) ml no contaminant BHR 22 10 6 3 1 1 12 −2 nocontaminant AHR 40 24 19 11 5 4 16 8 Total¹ +80 ppb NaCl BHR 27 13 10 62 1 14 −1 +80 ppb NaCl AHR 25 16 9 8 1 1 9 7 Total¹ +30 ppb OCMA BHR 6955 49 43 31 26 14 31 +30 ppb OCMA AHR 51 36 31 25 18 16 15 21 Total² +5ppb Lime BHR 26 14 10 6 2 1 12 2 +5 ppb Lime AHR 26 14 10 6 1 1 12 2Total¹ ¹Total fluid loss within 30 seconds ²Total fluid loss within 5minutes.

Upon comparison of the two sets of data, one of skill in the art shouldappreciate the that the weighting agent according the claimed subjectmatter has considerable fluid loss control properties when compared tothe API barite. Further it will be noted that the API barite also showssensitivity to drilled solids contamination whereas the new baritesystem is more tolerant.

Example 5

An experiment was conducted to demonstrate the ability of the newweighting agent to formulate drilling muds with densities above 20 poundper gallon [2.39 g/cm³].

Two twenty two pound per gallon [2.63 g/cm³].mud systems wereformulated, the weighting agents comprised a blend of 35% w/w new bariteweighting agent with 65% w/w API grade Barite (Fluid #1) weighting agentand 100% API grade barite (fluid #2), both with 11.5 pound per barrel[32.8 kg/n 3] STAPLEX 500 (mark of Schlumberger, shale stabiliser), 2pound per barrel [5.7 kg/m³] IDCAP (mark of Schlumberger, shaleinhibitor), and 3.5 pound per barrel [10 kg/m³] KCl. The other additivesprovide inhibition to the drilling fluid, but here demonstrate thecapacity of the new formulation to cope with any subsequent polymeradditions. The fluid was hot rolled to 200° F. (93.3° C.). Results areprovided in table VIII.

TABLE VIII Yield Viscosity (Fann Units) at various shear rates PointFluid (rpm of agitation: PV lb/100 ft² Loss 600 300 200 100 6 3 mPa · s(Pascals) ml Before Hot Rolling (#1) 110 58 46 30 9 8 52   6 (2.9) AfterHot Rolling(#1) 123 70 52 30 9 8 53  17 (8.1) 8.0 Before Hot Rolling(#2) 270 103 55 23 3 2 167 −64 (−32) After Hot rolling(#2) os 177 110 477 5 12.0 os: off-scale

Upon review of the above data one of skill in the art should appreciatethat the 100% API grade barite has very high plastic viscosity and is infact turbulent as demonstrated by the negative yield point. Further itwill be noticed that after hot rolling the rheology is so high it is offscale.

Example 6

This experiment demonstrates the ability of the new weighting agent inlow viscosity fluids (i.e. high fluidity formulations). The weightingagent is 100% colloidal barite according the claimed subject matter.Fluid #15 is a synthetic based drilling fluid (Ultidrill, Mark ofSchlumberger, a linear alpha-olefin having 14 to 16 carbon atoms). Fluid#16 is a water-based mud and includes a viscosifier (0.5 ppb IDVIS, Markof Schlumberger, a pure xanthan gum polymer) and a fluid loss controlagent (6.6 ppb IDFIO Mark of Schlumberger). Fluid #15 was hot rolled at200° F. (93.3° C.), fluid #16 at 250° F. (121.1° C.). After hot rollingresults are shown table IX.

TABLE IX Yield Viscosity (Fann Units) at various Gels¹ Point shear rates(rpm of agitation: PV lbs/100 ft² lbs/100 ft² 600 300 200 100 6 3 mPa ·s (Pascals) (Pascals) #15: 13.6 ppg 39 27 23 17 6 5 12 7/11 15 [1.63g/cm³] #16: 14 ppg 53 36 27 17 6 5 17 5/— 19 [1.67 g/cm³] A measure ofthe gelling and suspending characteristics of the fluid, determined at10 sec/10 min using a Fann viscosimeter.

Upon review of the above representative test data, one of skill in theart should appreciate that that the new weighting agents disclosedherein provide a way to formulate brine analogues fluids useful forslimhole applications or coiled tubing drilling fluids. Further it willbe noted that the rheology profile is improved by the addition ofcolloidal particles.

Example 7

An experiment was conducted to demonstrate the ability of the newweighting agent to formulate completion fluids, were density control andhence sedimentation stability is a prime factor. The weighting agent iscomposed of the new colloidal barite according to the claimed subjectmatter with 50 pound per barrel [142.65 kg/m³] standard API gradecalcium carbonate which acts as a bridging agent. The 18.6 ppg [2.23g/cm³] fluid was formulated with 2 pound per barrel [5.7 kg/m³] PTS 200(mark of Schlumberger, pH buffer) The static ageing tests were carriedout at 400° F. (204.4° C.) for 72 hours. Upon review of the exemplaryresults shown in the table below, one of skill in the art will note thatbefore (BSA) and after (ASA) static ageing a good stability tosedimentation and rheological profile can be achieved.

Viscosity (Fann Units) at various YP Free shear rates (rpm of agitation:PV lb/100 ft² water * 600 300 200 100 6 3 mPa · s (Pascals) ml 18.6 ppgBSA 37 21 15 11 2 1 16 5 (2.5) — 18.6 ppg ASA 27 14 11 6 1 1 13 1 (0.5)6 * free water is the volume of clear water that appears on top of thefluid. The remainder of the fluid has uniform density.

Example 8

This experiment demonstrates the ability of the new weighting agent toformulate low viscosity fluids and show it's tolerance to pH variations.The weighting agent is composed of the new colloidal barite according tothe claimed subject matter. The 16 ppg [1.91 g/cm³] fluid was formulatedwith caustic soda to adjust the pH to the required level, with thesubsequent fluid rheology and API filtration tested.

Yield Viscosity (Fann Units) at various Point Fluid shear rates (rpm ofagitation: PV lb/100 ft² Loss pH 600 300 200 100 6 3 mPa · s (Pascals)ml 8.01 14 7 5 3 7 0 (0)   8.4 9.03 14 8 5 3 6 2 (1)   8.5 10.04 17 9 63 8 1 0.5) 7.9 10.97 17 9 6 3 8 1 (0.5) 7.9 12.04 19 10 7 4 1 1 9 1(0.5) 8.1

Upon review of the exemplary results one of skill in the art shouldconcluded that a good stability to pH variation and rheological profileis established in the fluids formulated using the coated weightingagents disclosed herein.

Example 9

This experiment demonstrates the ability of the new weighting agent toformulate low rheology high temperature, high pressure stable water basefluids. The weighting agent is composed of the new colloidal bariteaccording to the claimed subject matter, with 10 pounds per barrel[28.53 kg/m³] CALOTEMP (mark of Schlumberger, fluid loss additive) and 1pound per barrel [2.85 kg/m³] PTS 200 (mark of Schlumberger, pH buffer).The 17 ppg [2.04 m³] and 18 ppg [2.16 g/cm³] fluids were static aged for72 hours at 250° F. (121° C.).

Yield Viscosity (Fann Units) at various Point Free Fluid Density shearrates (rpm of agitation: PV lb/100 ft² water Loss ppg PH 600 300 200 1006 3 mPa · s (Pascals) ml ml 17 7.4 28 16 11 6 1 1 12 4 (2) 10 3.1 18 7.542 23 16 10 1 1 19 4 (2) 6 3.4

Upon review of the above illustrative results, one of skill in the artshould appreciate that the fluids formulated in accordance with thepresent disclosure exhibit good stability to sedimentation and lowrheological profile with the subsequent filtration tested.

Example 10

The following example demonstrates that an oleaginous based drillingfluid containing the coated solids of the present invention gives betteroverall performance, with particular benefits in static sag, dynamic sagand fluid loss. Three fluids were formulated the first in accordancewith the teachings of the present disclosure, the second with fine grindNorway barite and EMI759 as a dispersant and a third fluid formulatedwith fine Chinese precipitated barite and EMI759 dispersant.

The following table provides the exemplary results obtained from theparticle size analysis using the Malvern Microplus instrument. Themeasurements were all taken in oil dispersant.

Particle Size Distributions Barite D₁₀ D₅₀ D₉₀ Dispersant 0.31 1.04 3.27coated barite Chinese 0.32 1.30 3.01 Precipitated Norway Fine 0.94 7.8631.25 Grind

One of skill in the art should appreciate that the Norway Fine Grindbarite would be considered to be on the fine end of the API standardgrind for barite. The fluids were formulated as indicated in thefollowing table. Each fluid was formulated to a density of 13 ppg and anoil to water (O/W) ration of 80/20.

Fluid Formulations Norway Precipitated Dispersant Barite Barite CoatedBarite Product (ppb) (ppb) (ppb) EDC99 As required As required Asrequired Chinese pptd Barite — As required — Norway fine Barite Asrequired — — OBWARP — — As required Poly-acrylate ester 7.5 7.5 —dispersant * Fatty acid amide 10 10 10 emulsifier Organoclay thickening3 3 3 agent Lime 6 6 6 CaCl₂ Brine (25 w %) As required As required Asrequired Gilsonite-base fluid 2 2 2 loss control additive * Note: Theamount added to the drilling fluid is equivalent to the amount ofcompound coated onto the dispersant coated barite.

The fluids were tested before and after aging at 250° F. The rheologieswere measured at 120° F. using a Fann 35 and the fluid loss values weremeasured at 250° F. The dynamic sags were measured on a Fann 35 at 120°F. after 30 mins at 100 rpm. The static sags were measured after agingthe fluid at 250° F. for 40 hours. The following table providesillustrative and exemplary data:

Fluid Properties - Dispersant Coated Barite Fluid +10% v/v Base +20 ppbHMP Seawater BHR AHR BHR AHR BHR AHR 600 31 37 39 47 40 44 300 17 20 2125 22 24 200 12 14 15 18 15 17 100 7 8 9 10 9 10  6 1 1 1 1 1 2  3 1 1 11 1 1 Gels 2/4 2/5 2/4 PV 14 17 18 22 18 20 YP 3 3 3 3 4 4 ES 731 810405 HTHP FL (250° F.) 2.2 1.2 1.2 Dynamic Sag Factor 0.501 — — StaticSag Factor 0.509 — —

Fluid Properties with Dry Powder Barites Added Norway Fine Grind BariteChinese Precipitated Barite +20 ppb +10% v/v +20 ppb +10% v/v Base HMPSeawater Base HMP Seawater BHR AHR BHR AHR BHR AHR BHR AHR BHR AHR BHRAHR 600 36 40 43 46 44 43 36 40 43 60 44 54 300 20 21 23 25 24 23 20 2224 32 24 30 200 13 1.4 15 17 16 16 15 16 17 22 17 21 100 7 8 8 10 9 9 89 10 12 10 12  6 1 1 1 1 1 1 1 1 1 1 2 2  3 1 1 1 1 1 1 1 1 1 1 1 1 Gels— 1/3 — 1/5 — 1/5 — 1/4 — 1/6 — 2/4 10′/10″ PV 16 19 20 21 16 20 16 1819 28 20 24 YP 4 2 3 4 8 3 4 4 5 4 4 6 ES 830 715 355 802 629 371 HTHPFL 2.0 3.6 1.8 8.8 12.0 — 250° F. Dynamic 0.527 — — 0.525 — — Sag FactorStatic Sag 0.718 — — 0.664 Factor

Upon careful review, one of skill in the art should appreciate that theresults demonstrate that each base fluid has similar rheologicalproperties, after clay and seawater contamination however, the fluidformulated with precipitated barite gives a greater increase in PV thanthe Norway barite and the fluids containing the dispersant coated weightmaterials of the present disclosure. This increase in plastic viscositymay be due to the ‘uncoated’ fines present in the fluid. Further such askilled artisan in comparing the fluid loss properties of the threefluids should see that the fluids containing the coated weighting solidsof the present disclosure show the best overall performance, althoughthe Norway barite also gives a similar performance. The precipitatedbarite fluid however, has a much poorer fluid loss. This is most likelydue to the very narrow particle size distribution of the precipitatedbarite as well as the coating effect of the barite particles being lesseffective by this process. A skilled person in the art of drillingfluids should note that the most significant difference between thethree fluids is in their sag performance. The fluid containing thecoated weight materials disclosed herein demonstrates very good sagproperties, both for dynamic and static aged sag. The coarser Norwayfine grind barite gave very poor sag performance, for both the dynamicand static sag tests. This is perhaps to be expected for an un-optimizedfluid using a coarser grind of barite, but it will also be appreciatedthat optimization to improve its sag performance will compromise the lowrheological properties of the fluid. The Chinese precipitated barite hasa much finer particle size distribution similar to the solids of thepresent disclosure, however its sag performance was also very poor forboth the dynamic and the static sag tests. This may also be due to theineffective coating of the barite by this process.

Upon further review of the above data one of skill in the art shouldconclude that in comparing the three different barites, similarrheologies are achieved despite their different particle sizes. Furtherit will be noted that the three base fluids give very similar rheologiesdemonstrates the benefits of a fluid formulated using the coatedcolloidal particles of the present disclosure. Further it will be notedthat fluids containing the solids of the present disclosure exhibitexcellent sag performance and fluid loss results compared to the otherfluids. This logically would lead a skilled person to conclude that inorder to formulate the other fluids to meet similar performance islikely to result in a much higher rheology fluid.

In view of the above disclosure, one of ordinary skill in the art shouldunderstand and appreciate that one illustrative embodiment of theclaimed subject matter includes a wellbore fluid having an oleaginousphase and an additive for increasing the density of the wellbore fluid.The additive comprises solid colloidal particles coated with adispersant. The dispersant is coated onto the colloidal particle duringthe comminution process of forming the particles. The illustrativeparticles have a weight average particle diameter (D₅₀) of less than 2μm and preferably a D₅₀ of less than 1.5 μm diameter. Preferably, thecolloidal particles are composed of a material of specific gravity of atleast 2.68. Exemplary starting materials for the colloidal particlesinclude many commonly known weighting agents including barite, calciumcarbonate, dolomite, ilmenite, hematite or other iron ores, olivine,siderite, and strontium sulfate as well as mixture and combinations ofthese and other similar weighting materials. The dispersant that iscoated onto the particle during the course of grinding is, in oneillustrative embodiment, selected from carboxylic acids of molecularweight of at least 150 Daltons. Alternatively, the dispersant coatingmay be made of compounds including oleic acid, polybasic fatty acids,alkylbenzene sulfonic acids, alkane sulfonic acids, linear alpha-olefinsulfonic acid or the alkaline earth metal salts of any of the aboveacids, and phospholipids as well as mixtures and combinations of thesecompounds. In another alternative and illustrative embodiment thedispersant is a polymeric compound, preferably a polyacrylate ester. Theillustrative polymeric dispersant should have an average molecularweight from about 10,000 Daltons to about 200,000 Daltons and morepreferably from about 17,000 Daltons to about 30,000 Daltons.

The claimed subject matter also encompasses a method of making anadditive for increasing the density of a fluid. In one illustrativeembodiment, the method includes comminuting a solid material and adispersant in a liquid medium, so as to produce solid colloidalparticles having a weight average particle diameter (D50) of less than 2μm that are coated with the dispersant. The liquid medium is preferablyan oleaginous fluid and more preferably an oleaginous liquid having akinematic viscosity less than 10 centistokes (10 mm2/s) at 40° C. and aflash point of greater than 60° C. Illustrative examples of sucholeaginous fluids include diesel oil, mineral or white oils, n-alkanesor synthetic oils such as alpha-olefin oils, ester oils orpoly(alpha-olefins) as well as combinations and mixtures of these ansimilar fluids. The dispersant that is coated onto the particle duringthe course of grinding is, in one illustrative embodiment, selected fromcarboxylic acids of molecular weight of at least 150. Alternatively, thedispersant coating may be made of compounds including oleic acid,polybasic fatty acids, alkylbenzene sulfonic acids, alkane sulfonicacids, linear alpha-olefin sulfonic acid or the alkaline earth metalsalts of any of the above acids, and phospholipids as well as mixturesand combinations of these compounds. In another alternative andillustrative embodiment the dispersant is a polymeric compound,preferably a polyacrylate ester. Optimally the illustrative dispersantis made of stearyl methacrylate, butylacrylate and acrylic acidmonomers. The illustrative polymeric dispersant should have an averagemolecular weight from about 10,000 Daltons to about 200,000 Daltons andmore preferably from about 17,000 Daltons to about 30,000 Daltons. Thesolid material may be selected from a wide variety of known weightingmaterials and in one illustrative embodiment the solid material isselected from the group consisting of barite, calcium carbonate,dolomite, ilmenite, hematite or other iron ores, olivine, siderite, andstrontium sulfate, mixtures and combinations of these and similarweighting materials that should be known to one of skill in the art. Inone preferred illustrative embodiment, the comminuting of the solidmaterial and the dispersant in the liquid medium is carried out in anagitated fluidized bed of a particulate grinding material.

While the apparatus, compositions and methods of this invention havebeen described in terms of preferred or illustrative embodiments, itwill be apparent to those of skill in the art that variations may beapplied to the process described herein without departing from theconcept and scope of the invention. All such similar substitutes andmodifications apparent to those skilled in the art are deemed to bewithin the scope and concept of the invention as it is set out in thefollowing claims.

1.-22. (canceled)
 23. A method of making a wellbore fluid, the methodcomprising: grinding a solid particulate material and a polymericdispersing agent to provide a resulting polymer coated solid material,wherein at least a portion of the resulting polymer coated solidmaterial has a particle diameter less than 2.0 microns, and wherein thepolymeric dispersing agent has a molecular weight greater than 10,000;and suspending the polymer coated wellbore additive colloidal solidmaterial in the wellbore fluid.
 24. The method of claim 23, wherein thepolymeric dispersing agent is a polymeric acrylate ester made from themonomers of stearyl methacrylate, butyacrylate, and acrylic acid. 25.The method of claim 23, wherein the grinding is carried out in thepresence of an oleaginous base fluid.
 26. The method of claim 23,wherein the solid material is selected from the group consisting ofbarite, calcium carbonate, dolomite, ilmenite, iron ores olivine,siderite, strontium sulfate, and combinations thereof.
 27. The method ofclaim 23, wherein greater than 25% of the polymer coated colloidal solidmaterial has a particle diameter less than 2 microns.
 28. The method ofclaim 23, wherein at least 60% of the polymer coated colloidal solidmaterial has a particle diameter less than 2 microns.
 29. A method ofmaking a wellbore fluid, the method comprising: grinding a solidparticulate material and a polymeric dispersing agent, such that thepolymeric dispersing agent coats the surface of the resulting colloidalsolid particles, wherein the resulting polymer coated solid material hasa weight average particle diameter of less than about 2 microns, andwherein the polymeric dispersing agent is a polymeric acrylate estermade from the monomers of stearyl methacrylate, butyacrylate, andacrylic acid; and increasing the density the wellbore fluid with theresulting polymer coated solid material.
 30. The method of claim 29,further comprising selecting the solid particulate material to havespecific gravity of at least 2.68.
 31. The method of claim 29, whereinthe solid material is selected from the group consisting of barite,calcium carbonate, dolomite, ilmenite, iron ores, olivine, siderite,strontium sulfate, and combinations thereof.
 32. The method of claim 29,wherein the grinding is carried out in the presence of an oleaginousbase fluid.
 33. The method of claim 32, wherein the base fluid isselected from the group consisting of: diesel oil, mineral oil, whiteoil, n-alkanes, synthetic oils, saturated and unsaturatedpoly(alpha-olefins), esters of fatty acid carboxylic acids andcombinations thereof.